Test instruments, such as oscilloscopes, have evolved to include substantially more memory than in the past. This increased memory has allowed such instruments to sample and store more data points of an acquired waveform. This large amount of data must be somehow displayed to allow a user analyze the acquired signal. A standard method of drawing a signal on screen is to draw a line joining each pair of adjacent points representing the captured signal. This method works fine with a small amount of data, but as soon as the number of data points increases, this method becomes very slow. This is due to the fact that drawing each line on the display device requires the main processor to access the hardware of the display device, a very slow operation as compared to the processor capability. The latest test instruments can record up to multiple millions of data values. Drawing multiple millions of lines on the display device can therefore become a very slow process and may take up to several seconds (an extremely long period of time). This procedure becomes particularly burdensome when the number of adjacent pixel pairs is substantially greater than the number of pixels that can be displayed on the screen. Not only does the display device need to be accessed for each line to be drawn, because of the lower display resolution, many of these lines will be drawn overlapping other previously or subsequently drawn lines in the same vertical pixel columns. Thus, the multiple, slow display access does not even result in the display of additional useful information to a user.
Traditionally, whether lines are drawn on the display between every adjacent pair of acquired data points or the lines are drawn after application of a compaction procedure, the rendering has been performed by drawing all lines on the display with same color and same intensity. This drawing procedure has two major negative side effects as compared to traditional rendering of data on an old analog display. It increases the import of visible noise on the rendered waveform, and also hides the internal structure of the rendered waveform, two things that are not issues with the analog display.
One such traditional method is shown in, for example, U.S. Pat. No. 5,255,365 and US Published Patent Application Number 20030107574, in which the data are compacted using a min/max method in order to draw vertical lines representative of the data on the display device. This method consists of examining every data sample that is to be drawn at the same vertical pixel column to find the minimum and maximum value of the data samples. After determining those min/max values, a single vertical line can be drawn in the pixel column instead of a multitude of overlapped lines. This technique is then applied to every pixel column in the display. The resulting display appears exactly the same as a display made by drawing a single line between every data sample (see FIGS. 2c, 3c, 4d). Because only a single line is drawn, there is no indication of the distribution of data values along that single line. Thus both methods (drawing each line and drawing a single line) have the disadvantage of hiding the internal statistical structure of the source signal. Also, because all data is drawn with the same intensity, each of the processes increases the visible noise of the measured signal.
Another traditional method that attempts to give a user additional information regarding the displayed information is show in U.S. Pat. No. 4,890,237. In this prior art method, the display of values in a spectrum analyzer is modified so that the intensity of each pixel element is a function of the number of samples at a particular power level within a predetermined frequency interval. While this shows the use of color to represent a third dimension, that of histogram values, in the prior art method, each of the histogram bins displayed on the display corresponds to a single frequency band. Thus, there is no consideration for compressing a number of y-axis values into a displayable number of values. Additionally, the spectrum analyzer inherently accumulates histogram values for its display. However, this prior art method gives no teaching how to display information that is acquired over time, and not previously accumulated into a histogram. Thus, while this prior art method is used to display histogrammed information, as will be discussed below, the present invention is concerned with using a histogram technique to display information that is not typically maintained or accumulated in a histogram.